Intra ocular lens   

Abstract: The invention relates to a intra lens (1) comprising a posterior surface and an anterior surface, said posterior surface having a curvature which is optimised for providing a minimal spherical aberration, wherein said curvature is optimized using the posterior chamber depth of an eye in which the IOL is to be inserted.

The present invention relates to an intra ocular lens (IOL) comprising a posterior surface and an anterior surface.

The emulation of human eye functionality is a challenge to modern medicine and technology. The human eye substantially has four optic elements: cornea, iris, crystalline lens and retina. The cornea provides a catching of a raw image of the environment. Due to the difference between the refractive index of air and of the cornea (nc≈1.376), the cornea with its refractive power of about 40 dpt contributes the main part of the refractive power of the eye. Next optical element in the human eye is the iris. The iris or iris diaphragm has two functions: (i) The regulation of light intensity, and (ii) regulation of depth of field or depth of focus. Its functioning is based upon a delicate interaction with the accommodation of the crystalline lens and thus provides good and clear vision of the healthy eye. Behind the iris, the crystalline lens provides the optical fine tuning in terms of precise imaging. Dependent on vision range, the crystalline lens varies its shape and in that way images the raw image, delivered by the cornea, to the retina precisely. This effect is referred to as accommodation. The ability of accommodation starts decreasing at an age of about 40 years and will usually be more are less lost at the age of about 60. With advancing age, the crystalline lens often becomes hazy, which process often ends in a more or less complete loss of vision, diagnosed as cataract. Finally, light will be received on the retina. The sensitivity of the retina is comparable to an ultra wide range optical film of about 10 to 40 DIN. It conducts electro-optical signals to the brain, where the image is interpreted.

It is common practise in IOL design to describe all these high precise and interacting functions by using a quite simple model consisting of two thick lenses. Furthermore, the crystalline lens exact purpose and function was long time underestimated. Maybe it is due to its smaller amount to refractive power (about 19 dpt) in comparison to the cornea, or because of the more or less complete loss of its dynamic function in case of cataract patients. Meanwhile, several variations in lens shape, i.e. equiconvex, biconvex, plano-convex, equiconcave, biconcave, plano-concave, meniscus and aspheric lens designs for enhancing the imaging quality are commonly in use. The state of the art for IOL design is the so called optimisation of IOL parameters by using ray tracing programs. This calculation is based on a two thick lens eye model, in which the lens shape is an input parameter.

Numerous optical models of the human eye have been developed up to now. Following L. Thibos (Thibos L N, Ye M, Zhang X, Bradley A., A new optical model of the human eye. Optics and Photonics News 1993; 4:12.), these models can conditionally be divided into anatomically-accurate models and analytical models. The goal of an anatomical model is to match gross anatomy of the human eye and to at least model the paraxial geometrical optics of the eye\'s thick lens system. Due to the complexity of implementation, an accurate anatomical model is hardly suitable for simulations of visual performance. To the contrary, by avoiding anatomical details, i.e. by treating the eye as an equivalent system of refracting surfaces with the appropriate aberrations, apertures, reflection and absorption coefficients, an analytical model can be obtained that ignores the eye anatomy completely. Analytical models are suitable in cases when the optical performance of the eye should be estimated with high accuracy irrelevant to its real structure. An acceptable compromise is a simplified anatomical model that comprises a physically correct description of the eye, including its dimensions and optical properties. Typical parameters of the human eye can be found in many publications, e.g. in OSA Handbook of Optics or in the well-known description of the “standard military eyeball” model MIL-HDBK-141. References to several widely-used physical models of the eye are cited in: Liou H L, Brennan N A, “The prediction of spherical aberration with schematic eyes”, Ophthalmic Physiol Opt 1996; 16:348-54, in Thibos L N, Ye M, Zhang X, Bradley A., “The chromatic eye: a new reduced-eye model of ocular chromatic aberration in humans”, Appl Opt 1992; 31:3594-600, in R. Navarro, J. antamaria, and J. Bescos, “Accommodation-dependent model of the human eye with aspherics”, Instituto de Optica, Serrano 121, 28006 Madrid, Spain, in Larry N. Thibos and Arthur Bradley, “Modelling the Refractive and Neuro-Sensor Systems of the Eye”, School of Optometry, Indiana University. Bloomington, Ind. 47405, and in Atchison & Smith, “Optics of the Human Eye”, Butterworth & Heinemann publisher 2000.

In patent literature, many so called aberration free IOL designs are described. Reference is made to EP1850793, EP1857077, WO2004/090611 and US2006279697, which are incorporated by reference as if fully set forth. All of them describe the IOL shapes with either one or both surfaces of the IOL shapes to be aspheric and the shape is defined by a conic constant. These IOL shapes are optimised by ray tracing or iteration using a certain eye model, while considering the specific optical condition behind the cornea. This method implicitly assumes that the retina is located at the back focal plane of the optical system.

SUMMARY

The invention aims provide a method for designing an intraocular lens, and a resulting IOL, that contributes almost zero or a pre-specified amount of spherical aberration to a wave front passing through the IOL.

According to a first aspect of the invention this is realized with an intra ocular lens (IOL) comprising a posterior surface and an anterior surface, said posterior surface having a curvature which is optimised for providing a predefined spherical aberration, wherein said curvature is optimized using the posterior chamber depth (PCD) of an eye in which the IOL is to be inserted.

The invention furthermore provides a method for producing a customized IOL for an eye of a person, said IOL having an anterior side and a posterior side, said method comprising the steps of: Measure at least one biometric value of said eye of said person from which a posterior chamber depth (PCD) of said eye can be derived; calculating the curvature of said posterior surface for said IOL for said person using said measured biometric value; producing said IOL having said posterior surface curvature.

The invention provides a method which enlarges the inflexible two thick lens eye model and which introduces shape fine tuning for IOL design taking into account the optical properties of the crystalline lens separately. The invention is therewith of such precision, that not only a preferred optical implant can be designed for cataract patients, but also for patients having presbyopic eyesight. The invention uses measurable parameters, the optical power and the conic constant. The refractive power of the cornea and the position of the crystalline lens behind the cornea lead to a manufacturer specific “A” constant of an IOL. However, for shape fine tuning of the IOL, the lens is considered as detached optical device, separated from the two lens model. Shape fine tuning is done by taking into account third order aberration, the higher order aberration terms might be taken into consideration. The third order aberration terms are in geometric optics also known as “Seidel” aberrations, whereas the spherical aberration effects the imaging at most optical modelling of finite element surface displacements using commercial software, see Keith B. Doyle, Victor L. Genberg, Gregory J. Michels, Gary R. Bisson, SPIE publication Sigmadyne Inc., 803 West Avenue, Rochester, N.Y. 14611. Thus the invention focuses on the effect of spherical aberration. Using the calculations mentioned above, taking into account posterior chamber depth and directed spherical aberration, the IOL power and the conic constant are determined.

It was found that for a biconvex IOL situated in a human eye, the spherical aberration generated by the posterior surface is many times bigger than the spherical aberration generated by the anterior surface.

For a biconvex IOL or more general an IOL with significant negative curvature of the posterior surface, it is found that in most cases it is sufficient to correct for the spherical aberration resulting from the posterior surface only.

Using the current method, it is also possible to correct for the anterior surface by taking in account the optical power of the posterior surface.

The amount of spherical aberration generated by the posterior surface of a biconvex lens can be corrected on the posterior surface itself or on the anterior surface.

The amount of spherical aberration generated by the anterior surface of a biconvex lens can be corrected on the anterior surface itself or on the posterior surface.

The posterior chamber depth can be found by direct measurement by using for example a commercial available instrument to perform the biometry of the eye such as the IOLMaster, Zeiss Germany, Lenstor LS900, Haag Streit, Switzerland or a Acoustic method called A-scan or by applying statistical found correlations between the required optical power of the IOL and the posterior chamber depth. By examination of large sets of biometric data, surprisingly a strong correlation, in fact, a linear correlation, was found between the calculated required optical power for the IOL implant and the posterior chamber length. The method according this invention integrated into the biometry software of the above mentioned instruments will pave the way to efficient and fast customized IOL calculation without complicated ray tracing and use of model eyes. The system would then calculate the specific conic constant or coefficient needed to minimize the spherical aberration for said IOL for said patient.

In an embodiment, the curvature is a mathematical function of the PCD.

In an embodiment, the mathematical function is defined as or is equivalent to

z = r 2 / R 2 1 + 1 - ( 1 + k )  r 2 / R 2 ,

with

k = n 2  R 3 n 1 2  ( 1 R - 1 PCD ) 2  ( n 1 + n 2 PCD - n 2 R )

in case of zero additional spherical aberration or, when a specified amount of spherical aberration should be added, with

k = n 2  R 3 n 1 2  ( 1 R - 1 PCD ) 2  ( n 1 + n 2 PCD - n 2 R ) +

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